A mesoscale study on the thermodynamic effect of stress on martensitic transformation
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I.
INTRODUCTION
THE thermomechanical behavior of polycrystalline materials undergoing a solid-solid phase transformation is very complex. In addition to the change in the crystal lattice going hand in hand with a jump in physical properties, elastic-plastic deformations caused by microstructural stresses have an effect on the kinetics of transformation. The need for understanding and predicting the mechanical behavior of solids undergoing a phase transformation is motivated by the desire for a predictive capability to assist alloy-design efforts for steels by utilizing the mechanical properties of the phase transformation itself. The spatial scale relevant to the mechanical behavior extends from atomistics over dislocation interface and microstructure up to the continuum level, t~j In the case of martensitic transformation (MT), the progress of the overall transformation has been studied on the dislocation-interface level under the aspect of stressassisted ~2J and strain-induced nucleation, t3~ Constitutive relations for transformation-induced plasticity (TRIP) have been derived from transformation kinetic theories, t41 In the statistical models derived from these theories, t21 the progress of overall transformation is seen to be governed by a potency distribution of strain embryos formed at various defects. Microstructural stresses in polycrystals arise both because of the deformation incompatibilities and anisotropic thermal expansion of the constituent crystallites, as well as because of the accommodation of incompatible strain fields emerging from the shape change associated with martensitic transformation. F. MARKETZ, Research Assistant, Christian Doppler Laboratory for Micromechanics of Materials, and F.D. FISCHER, Professor, the Institute of Mechanics and Christian Doppler Laboratory for Micromechanics of Materials, are with the University for Mining and Metallurgy, A-8700, Loeben, Austria. Manuscript submitted July 28, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
Transformation-induced plasticity is attributed to two different mechanisms: on one hand, to stress-assisted nucleation of martensitic-crystallographic variants favorably aligned to the direction of applied stress (orientation effect or Magee effect) and, on the other hand, to plastic deformation of the parent- and product-phase microconstituents (Greenwood-Johnson effect). In steels, these two effects are interacting. The Magee effect is clearly evident with stress-induced martensitic transformation in monocrystals of shape-memory alloys.lSl Even in steels, a remarkable macroscopic deformation of a specimen transforming in an externally applied stress field is obtained, although the sample is loaded below the yield stress of the parent phase in order to exclude classical plasticity. Transformation-induced plasticity has been investigated experimentally in anisothermal "creep" and tensile tests by Gautier eta/. [6,7,81 Because from these experiments it is difficult to determine corresponding contributions to overall deformations from the eff
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